missing purchased song

If it's no longer in the store you won't be able to redownload it, you will have to copy it back from your backup of your downloads/computer or back from an iPod or iOS device if you have it on one

If you can't find it in your country's store (if you've only checked the Purchased redownloading area check the main part of the store, and if is there check to see if you've hidden it from Purchased : Hide and unhide music, movies, TV shows, audiobooks, and books - Apple Support) it's likely that the rights-holder has removed it, in which case you won't be able to redownload it (unless they put it back in the store)